MONROVIA, Liberia — Liberia released its last Ebola patient, a 58-year old teacher, from a treatment center on Thursday, beginning its countdown to being declared Ebola free.

"I am one of the happiest human beings today on earth because it was not easy going through this situation and coming out alive," Beatrice Yardolo told The Associated Press after her release. As she walked out of the clinic, wearing a bright yellow shirt, hospital cap and a beaming smile, Yardolo kept thanking God and the health workers at the center.

Yardolo said she had been admitted to the Chinese-run Ebola treatment center in the Paynesville district of Monrovia on Feb. 18. A mother of five, she is originally from the northeastern county of Nimba near the borders with Guinea and Ivory Coast, but lives in Monrovia where she teaches English at a church-run school.

The St. Paul's Bridge community where she resides and works had become the last "hotspot" for Ebola cases in Monrovia, according to Tolbert Nyenswah, Assistant Health Minister and head of the country's Ebola response.

Yardolo's release brings Liberia hope after about a year of battling the deadly Ebola virus, which killed more than 4,100 people in this West African country.

There are no other confirmed cases of Ebola in the country, and as such Liberia can begin to count up to 42 days to be declared Ebola free in keeping with World Health Organization protocols and standards, Nyenswah said Wednesday. He challenged all Liberians to commit themselves to achieving "zero Ebola infections" by rigidly abiding by the anti-Ebola regulations.

Liberian President Ellen Johnson Sirleaf has said no country can be declared Ebola free until all the other countries have no cases.

Sierra Leone, Liberia and Guinea have been hardest hit in the yearlong Ebola outbreak, which is estimated to have left more than 9,800 people dead.

"This is an encouraging sign for Liberia. However, there is no room for complacency as the number of new Ebola cases in the region has risen this week," said Vickie Hawkins, director of Medecins Sans Frontieres UK. "People move easily over the porous borders that separate Guinea, Liberia and Sierra Leone, so until 42 days pass without a new case in any of the three worst affected countries we need to remain vigilant."

The WHO on Wednesday reported 132 new Ebola cases last week, an increase from the 99 cases reported the previous week. The agency said the spread of Ebola remains "widespread" in Sierra Leone and noted that cases have jumped both there and in Guinea.

Nine new cases were reported in a 24-hour period, according to an update from the Sierra Leone government on Tuesday.

"There is always the risk of re-infection. We're still seeing a lot of suspected and probable cases in Liberia," said Sebastian Funk, an Ebola expert at London's School of Hygiene and Tropical Medecine. "Ebola was first introduced to Liberia from Guinea and I see no reason why that wouldn't happen again if it is still circulating in Guinea."

Funk said that continued resistance to aid efforts in Guinea could be driving the spread there.

According to WHO data, both Guinea and Sierra Leone have recorded more than a dozen unsafe burials in the past several weeks, while Liberia hasn't reported any unsafe burials since mid-February. Unsafe burials, where friends and family members are in close contact with the body of an Ebola victim, have often sparked explosive spread of the disease during the outbreak.

Liberia has also successfully traced all known contacts of Ebola cases, compared with just 49 percent for Guinea and 78 percent for Sierra Leone. Not being able to trace potential cases means health officials in Guinea and Sierra Leone don't know where many new cases are coming from.

Resistance to Ebola containment efforts also continues to be a problem in Guinea and Sierra Leone, which have both reported numerous "security incidents" or other forms of refusal to cooperate. Since late February, Liberia has only reported just one such event.

The U.N. health agency said Thursday it will start large-scale testing of an experimental Ebola vaccine in Guinea on Saturday to see how effective it might be in preventing future outbreaks of the deadly virus.

The health agency's vaccination strategy in Guinea aims to create a buffer zone around an Ebola case to prevent its further spread. Officials will vaccinate people who have already been exposed to Ebola cases and are at risk of developing the disease.

The vaccine being tested — VSV-EBOV — was developed by Canada and is now licensed to Merck. A second vaccine — one developed by U.S. National Institutes of Health and GlaxoSmithKline — will be tested in a separate study as supplies become available.

Associated Press Medical Writer Maria Cheng in London contributed to this report.